Pharmacokinetics of thiopental sodium in patients undergoing Hyperthermia treatment was studied to understand why these patients need an unusually large dose of the sedative. HPLC-methods were developed to assay pentothol in plasma and in its metabolite in urine. The results suggest that the plasma levels of thiopental in hyperthermia patients are within normal range in spite of the high dose, there is no unusual increase in the metabolism and drug levels stay high even after 24 hrs. of dosing, all suggesting that the drug gets shunted into fat reservoirs in the body during hyperthermia.